The invention relates to data transmission in a telecommunication system, particularly in a case where the maximum data rate of a traffic channel is equal to one user data rate at a terminal interface.
Mobile systems generally mean different telecommunication systems that enable private wireless data transmission for subscribers moving within the system. A typical mobile system is a public land mobile network PLMN. The PLMN comprises fixed radio stations (base stations) located in the service area of the mobile network, the radio coverage areas (cells) of the base stations providing a uniform cellular network. A base station provides a radio interface (air interface) in the cell for communication between a mobile station and the PLMN.
Another area of mobile systems includes satellite-based mobile services. In a satellite system, radio coverage is obtained by satellites instead of terrestrial base stations, the satellites being in orbit round the earth and transmitting radio signals between mobile stations (or user terminals UT) and land earth stations (LES).
Subscriber mobility requires similar solutions in satellite mobile systems as in the PLMNs, i.e. subscriber data management, authentication and location management of mobile subscribers, handover, etc. The satellite systems should also support similar services as the PLMNs.
One way of meeting the above requirements in satellite mobile systems is to use existing PLMN solutions. In principle this alternative is very straightforward since a satellite system can basically be compared to a base station system of a mobile system having a different radio interface. In other words, it is possible to use conventional PLMN infrastructure, where the base station system(s) is(are) a satellite system. In such a case, the same network infrastructure could, in principle, even contain both conventional PLMN base station systems and satellite xe2x80x98base station systemsxe2x80x99.
There are many practical problems, however, in adaptation of PLMN infrastructure and a satellite system. A problem apparent to the Applicant is that a PLMN traffic channel and a traffic channel of a xe2x80x98radio interfacexe2x80x99 in a satellite system differ considerably. Let us examine an example where the PLMN is the pan-European digital mobile system GSM (Global System for Mobile Communication), and the satellite mobile system is the Inmarsat-P system currently developed.
At present, a GSM traffic channel supports data transmission at user rates 2400, 4800, 7200 and 9600 bit/s. In addition to user data, status information on the terminal interface (control signals of a V.24 connection) is transmitted in both directions on the traffic channel. In transparent HSCSD data service, it is also necessary to transfer synchronization information between subchannels. On the traffic channel is used channel coding to reduce the effect of transmission errors. The channel coding and the above-mentioned additional information increase the bit rate at the radio interface above the actual user rate. The user rates 2400, 4800 and 9600 bit/s are corresponded to by rates 3600, 6000 and 12000 bit/s at the radio interface.
The Inmarsat-P satellite system requires that standard data rates up to 4800 bit/s (e.g. 1200, 2400, 4800 bit/s) can be transferred on one traffic channel, and that standard data rates exceeding 4800 bit/s (e.g. 9600, 14400, 19200 bit/s, etc.) can be transferred by using several parallel traffic channels, like in the HSCSD service of the GSM system.
In the Inmarsat-P satellite system, the data rate of one traffic channel at the radio interface is at most 4800 bit/s, which equals the user data rate of 4800 bit/s at the terminal interface. In a data service employing two traffic channels the data rate at the radio interface equals the user data rate of 9600 bit/s at the terminal interface. A problem arises when not only the user data but also the above-described terminal interface status information and potential inter-subchannel synchronization information should be transferred over the radio interface. Therefore the protocol data unit, i.e. frame structure, used by the satellite system at the radio interface should be defined to carry the above-mentioned control and synchronization information over the radio interface. One way would be to use a GSM solution, i.e. a V.110-based frame structure, also at the radio interface of the satellite system. However, this would be a very complicated solution, and it would significantly reduce the user data rates available. A single traffic channel could not support the user data rate of 4800 bit/s since a V.110 frame structure and the terminal interface status information increase the actual data rate (radio interface rate) above 4800 bit/s. Therefore the highest standard user data rate on one traffic channel would be 2400 bit/s. For the same reason, a two-traffic-channel data service could not support the user rate of 9600 bit/s, but the highest standard user data rate would be 4800 bit/s (or in some systems 7200 bit/s). A corresponding decrease in the available data rates would also occur in data services employing more than two traffic channels. Such a solution, where the overhead information causes a significant loss of capacity, would not be satisfactory.
A similar problem can also arise when other types of radio interfaces, such as wireless telephone systems, are connected to the PLMNs.
A similar problem can also arise with other types of connections in which the radio interface rate is to be used as effectively as possible. For example, a new 14400 bit/s traffic channel has been planned for the GSM. In order that the status information of the terminal interface and any other control information could be transferred over the radio path in addition to the 14400 bit/s user data, the radio interface rate, implemented on the present principles, would be higher than 14400 bit/s, about 18 kbit/s. A higher radio interface rate requires that the existing radio networks should be re-designed and the intermediate rate (TRAU) increased so that only two subchannels could be put in a single 64 kbit/s timeslot in the HSCSD service (i.e. the efficiency decreases in a TRAU data link). The radio interface rate of 14400 bit/s does not cause such problems, but the actual user data rate would then be below 14400 bit/s, if the new traffic channel were implemented on the same principles as the existing GSM traffic channels. It would thus be preferable to implement a user data rate of 14400 bit/s at a radio interface rate of 14400 bit/s.
An object of the present invention is to provide a solution that supports transmission of user data, terminal interface status information and any other control or synchronization information through a transparent traffic channel having a data rate that is equal to the user data rate at the terminal interface.
The object of the invention is a method according to claim 1, and equipment according to claim 8.
In the present invention, the terminal interface status information and any other control or synchronization information are transferred through the traffic channel in redundant data elements of end-to-end protocols, e.g. in redundant parts of protocol data units of user data or in start and stop bits of asynchronous data characters. Overhead information does thus not increase the number of bits transmitted, so the data rate of the traffic channel can be equal to the user data rate at the terminal interface. In high-speed data transmission, a data connection may comprise a group of two or more traffic channels, whereby the overall data rate of the traffic channel group may be the same as the user data rate at the terminal interface.
Although the above solution usually works well, the operation can be optimized in situations where no user data elements of end-to-end protocols are present at the terminal interface and yet e.g. the status bits of the user interface have to be transferred through a traffic channel. These situations include, for example, call set-up, a pause in data transmission, and call set-down.
In the present invention, it is monitored at the transmitting end whether there are user data elements of the end-to-end protocol at the user interface. If user data elements of the end-to-end protocol are lacking and e.g. a change in the status information has to be sent, transmission of a bit stream through a traffic channel is interrupted, and specific auxiliary frames that carry the status bits and any other additional information are sent instead. At the receiving end, the status bits and other additional information are separated from the auxiliary frames and replaced with a suitable bit stream, which is supplied to the terminal interface. Simultaneously the terminal interface is monitored, and if new user data elements are received, the routine switches to an operating mode where the status bits and any other additional information are sent in the redundant parts of the user data elements. The type of the auxiliary frame can be selected specifically for each application, as long as both the transmitting end and the receiving end know what type of auxiliary frame is used.